Derflinger (Pronunciation: /dɛrfliŋgɚ/) is an old sword that Louise bought for Saito. Louise spent most of her money in order to pay for an expensive healing potion for Saito after a fight early in the series, forcing her to buy Derflinger due to its cheap price of 100 Écu. Derflinger has the ability to speak and is seen to be very intelligent, usually offering useful information at times. (Other Names Gandalfr)
Summary
"Derf", as called by Saito, provides information on whatever subject he may remember. Despite being a legendary sword, in the light novels Derflinger often forgets some information due to being an old sword. In the first season, he is always called 'annoying' by Saito and Louise. However, as the series progresses, he is proven to be very useful, being able to recognize Louise as a Void Wizard, Tiffania as an Elf (but possibly, he may also know that she is a Void) and was also able to deduce Sylphid as a Rhyme Dragon (Illococoo) in disguise, as well as Bidashal the Elf and the Jormungandr's Counter ability (that can only be penetrated by Void magic and/or strong enough physical force, which Earth weaponry is vastly superior compared to Halkeginian weaponry).
Derflinger has the ability to absorb spells, but more powerful spells like those of a Square-class mage takes longer to absorb. He can also take control of his wielder and move them to safety if the wielder faints, but this uses up his own store of magic (Willpower). And in Volume 19, it is revealed if his physical form is destroyed he can move onto another blade in the vicinity to survive with all his abilities, though this takes a while.
Notably, Derflinger is known in the second season as someone who either Louise or Saito come to for romantic advice (He was the reason for Louise to ask Saito to marry her) despite being only a magical item that possibly cannot experience feelings of love. But this is proven to be false since he feels happy when with Saito, sad when he learned of his death (until Tiffania cured him) or just teases Louise about her feelings for Saito whenever possible.
In the novels, when assassins were hired to kill Saito in Volume 16, he was shattered by one of the assassins. In Volume 19, it was revealed Derflinger is a 'spirit sword' made by the first Gandalfr, Sasha, using elven magic and was the same blade that pierced Brimir's heart and lead to his death. And the consciousness of the sword can hop between blades if necessary when the old vessel is destroyed. Currently, Derflinger resides in one of Saito's katanas.
In episode 10, season 4 Derflinger tries to absorb the attack from the Ancient Dragon, but was overpowered and soon shattered after the tail end of the blast. His shattering proves strong enough to knock Saito off of the dragon they were flying on.
Fortunately in episode 12, season 4 it is discovered that Derflinger is still alive. Saito finds him in the control panel of the fighter jet. He believes that his consciousness was transported to Saito's Runes upon his destruction. Both of them are happy to meet each other again.